CVE-2026-64306: Linux Kernel CTR_DRBG Vulnerability

CVE-2026-64306 Overview

CVE-2026-64306 is a vulnerability in the Linux kernel's cryptographic Deterministic Random Bit Generator (DRBG) subsystem. The flaw resides in the drbg_ctr_generate() function of the CTR_DRBG implementation. Under certain failure conditions, the function returns success while leaving the output buffer uninitialized. Callers that trust the return value will consume uninitialized kernel memory as if it were cryptographically secure random data. This can weaken cryptographic operations that depend on the DRBG for key material, nonces, or initialization vectors, and can also lead to information disclosure of kernel memory contents.

Critical Impact

Cryptographic consumers may receive uninitialized kernel memory in place of secure random output, undermining confidentiality guarantees of dependent operations.

Affected Products

• Linux kernel (crypto subsystem, CTR_DRBG implementation in drbg.c)
• Distributions shipping affected stable kernel branches prior to the linked fix commits
• Any userspace or kernel component consuming DRBG output for cryptographic operations

Discovery Timeline

• 2026-07-25 - CVE-2026-64306 published to NVD
• 2026-07-25 - Last updated in NVD database

Technical Details for CVE-2026-64306

Vulnerability Analysis

The vulnerability is classified as an Uninitialized Memory Use issue within the Linux kernel crypto layer. The CTR_DRBG (Counter mode Deterministic Random Bit Generator) is defined by NIST SP 800-90A and is used across the kernel to produce cryptographically secure pseudorandom output. The kernel implementation exposes this generator through drbg_ctr_generate(), which is expected to either populate the caller's output buffer with fresh random bytes or return a non-zero error code on failure.

The defect breaks that contract. On specific internal failure paths, drbg_ctr_generate() returns 0 (success) without writing to the output buffer. Callers therefore treat whatever residual data occupies the buffer as valid DRBG output. Depending on the caller, that data may be used as a symmetric key, an IV, a nonce, or seed material for higher-level protocols.

The practical impact is twofold. First, cryptographic operations built on the affected output lose their security assumptions, since the underlying bytes are predictable stack or heap contents rather than random. Second, when the uninitialized buffer is subsequently transmitted or exposed, previously used kernel memory can leak to user space or across trust boundaries.

Root Cause

The root cause is a missing propagation of an internal failure result inside drbg_ctr_generate(). Error branches exit without setting a non-zero return code and without zeroing or filling the output buffer, so the function reports success while the buffer remains uninitialized. The upstream fix, applied across multiple stable branches in commits 074db6db03a0, 23b8b188cb32, 39a31ad9e2a5, 75597e8774f3, 7b03312491f9, a9e886f73dd7, bbbac12083ef, and cc42fb40171c, corrects the error handling so failures are reliably surfaced to the caller.

Attack Vector

This is not a directly remote-exploitable memory corruption bug. Exploitation requires an attacker to induce or observe a failure inside CTR_DRBG generation, then consume the resulting cryptographic output through a legitimate interface such as /dev/random consumers, kernel TLS, dm-crypt, IPsec, or in-kernel key generation. Because the output is silently invalid, an attacker with visibility into ciphertext, key material, or leaked buffers can gain an advantage against protocols that assumed strong randomness. Local unprivileged users are the most realistic threat actors, since they can more readily trigger and observe crypto operations.

No public proof-of-concept code is available. See the linked kernel commits for the technical details of the fix.

Detection Methods for CVE-2026-64306

Indicators of Compromise

• No network-level indicators exist for this vulnerability; exploitation is local and does not produce distinctive traffic signatures.
• Kernel logs may contain crypto self-test failures or DRBG health-test warnings on affected systems, though absence of logs does not rule out the issue.
• Unexpected repetition or low entropy in cryptographic material produced by kernel consumers of CTR_DRBG can indicate that uninitialized output has been returned.

Detection Strategies

• Inventory running kernels and compare against the fixed stable branches referenced in the upstream commits 074db6db03a0, 23b8b188cb32, 39a31ad9e2a5, 75597e8774f3, 7b03312491f9, a9e886f73dd7, bbbac12083ef, and cc42fb40171c.
• Use configuration and vulnerability management tooling to flag hosts whose uname -r output maps to unpatched kernel versions.
• Review whether CTR_DRBG is selected as the active DRBG via /proc/crypto and prioritize patching for systems where it is in use.

Monitoring Recommendations

• Forward kernel messages (dmesg, journald) to a centralized log platform and alert on drbg, crypto, and FIPS self-test error strings.
• Monitor package management events to confirm that kernel updates addressing CVE-2026-64306 are installed and that hosts have rebooted into the fixed kernel.
• Correlate patch state with cryptographic workload telemetry so that hosts running TLS, VPN, or disk-encryption services are prioritized for remediation tracking.

How to Mitigate CVE-2026-64306

Immediate Actions Required

• Upgrade to a Linux kernel version that includes one of the fix commits listed in the upstream references, then reboot affected hosts.
• For distribution-managed systems, apply the vendor kernel update that references CVE-2026-64306 as soon as it is available.
• Rotate long-lived keys, certificates, and other secrets that may have been generated by kernel consumers of CTR_DRBG on unpatched systems, especially where local untrusted users had access.

Patch Information

The issue is resolved upstream in the Linux kernel crypto subsystem. The following commits contain the fix across stable branches: 074db6db03a0, 23b8b188cb32, 39a31ad9e2a5, 75597e8774f3, 7b03312491f9, a9e886f73dd7, bbbac12083ef, and cc42fb40171c.

Workarounds

• Where feasible, configure the kernel crypto API to prefer a non-CTR DRBG (for example HMAC_DRBG or Hash_DRBG) until the fixed kernel can be deployed.
• Restrict local access on multi-tenant systems to limit which users can trigger DRBG-consuming operations while patching is pending.
• Reissue cryptographic material after patching so that any keys or IVs derived from potentially uninitialized output are no longer in service.

# Verify current kernel and active DRBG selection
uname -r
grep -A2 'drbg' /proc/crypto

# Debian/Ubuntu: install and boot the fixed kernel
sudo apt update && sudo apt install --only-upgrade linux-image-generic
sudo reboot

# RHEL/CentOS/Fedora: apply the vendor kernel update
sudo dnf update kernel
sudo reboot